The present invention relates to optical modulators, and more specifically to a technique for linearizing the output of an external optical intensity modulator.
Recently, there has been a growing interest in the development of analog, amplitude modulated optical communication systems. In comparison with digital systems, analog communication systems provide an efficient use of bandwidth. This is particularly useful in cable television (CATV) transmission system applications, where it is necessary to transmit a large number of video channels through an optical fiber. Compatibility with existing equipment is achieved by using the same signal format for optical transmission that is in use for coaxial cable signal transmission.
In order to transmit an information signal (e.g., a television signal) over an optical fiber, a light beam ("carrier") must be modulated with the information signal. The "electrooptic effect" has been advantageously used to provide modulators for this purpose. For example, electrooptic modulators using miniature guiding structures are known which operate with a low modulating power.
In electrooptic modulators, the electric field induced linear birefringence in an electrooptic material produces a change in the refractive index of the material which, in turn, impresses a phase modulation upon a light beam propagating through the material. The phase modulation is converted into intensity modulation by the addition of polarizers or optical circuitry. Ideally, an electrooptic modulator should have a linear relationship between its output optical power and the applied modulating voltage.
In a "Mach Zehnder" type electrooptic modulator, an optical carrier (laser beam) is split into two paths. At least one path is electrically phase modulated. The two signals are then recombined in an interferometer to provide an intensity modulated carrier. Typically, lithium niobate (LiNbO.sub.3) is used as the electrooptic material. Waveguides in such materials are readily formed by titanium indiffusion.
The output power curve of a Mach Zehnder modulator is nonlinear. Practical analog optical communications systems, however, demand a high linearity. See, for example, W. I. Way, "Subcarrier Multiplexed Lightwave System Design Considerations for Subscriber Loop Applications", J. Lightwave Technol., Vol. 7, pp. 1806-1818 (1989). Modulator nonlinearities cause unacceptable harmonic and intermodulation distortions. When it is necessary to communicate a large number of channels, as in a CATV application, intermodulation distortions ("IMD") can impose serious limitations on the system performance.
The problem of intermodulation distortions, and particularly second order distortion, is complicated by the fact that lithium niobate is an inherently unstable material. Thus, it is not possible to simply bias a Mach Zehnder modulator (e.g., to operate at its quadrature point) and then expect the modulator to continue to run at the desired operating point. In fact, the bias condition will dynamically change due to factors such as temperature, photorefractive instability, and displacement currents within the lithium niobate material. Thus, additional steps must be taken to ensure that the external modulator will run over time without an increase in second order distortion.
It would be advantageous to provide a technique that dynamically tracks the second order distortion performance of an external optical modulator and maintains such performance at an acceptable level. It would be further advantageous to provide such a technique that can be used with a single optical modulator or a plurality of such modulators cascaded in series. The present invention provides a method and apparatus enjoying the aforementioned advantages.